1. Field of the Invention
The present invention relates to a method of supporting handover in a wireless access system.
2. Discussion of the Related Art
Hereinafter, handover and a ranging process will be described in brief.
Handover means that communication which is being performed is maintained as it is when a mobile station goes out of a cell zone of a serving base station and enters a cell zone of another base station. In other words, the handover means that exchange of channel or line is performed to maintain call which is being performed.
Handover can be divided into softer handover, soft handover, and hard handover.
Softer handover is handover performed within one cell, and means that a mobile station converts a current channel used within cell coverage to an excellent channel. Soft handover means that two neighboring channels are managed at the same time and one of the channels is finally disconnected. Soft handover can easily be implemented as the same frequency band is used in a CDMA mode. Hard handover means that a mobile station disconnects a current channel in which communication is being performed, and directly connects another channel.
The mobile station performs a ranging procedure after moving a cell zone of a base station currently in service to another cell zone of another base station and performing handover to another base station.
Ranging means a set of processes for maintaining connection quality (particularly, synchronization) of radio frequency (RF) communication between a base station and a mobile station in IEEE 802.16 or mobile Internet. Particularly, during OFDMA/TDD based multiple access, since a base station transmits data to several mobile stations based on one reference timing in a downlink, there is no problem in the downlink. However, since several mobile stations respectively transmit a signal in an uplink, radio wave delay may occur differently for each of the mobile stations. In this respect, as a separate timing synchronization method, a ranging procedure is performed between the base station and the mobile station. Namely, ranging is a procedure for exactly controlling synchronization of transmission time in several mobile stations.
For power control, and synchronization acquisition of an uplink between the mobile station and the base station, initial ranging, periodic ranging and hand-off (HO) ranging can be used. Also, bandwidth request ranging can be performed by the mobile station to request the base station of bandwidth.
Initial ranging is used for initial network entry and connection when the mobile station initially enters the base station (for example, when the mobile is powered on or when a long-term idle mode is converted into an active mode). Periodic ranging is periodically performed by the mobile station for synchronization tracking towards the base station. Handoff ranging is performed by the mobile station for synchronization with the base station during handoff. Bandwidth request (BR) ranging is performed to request uplink allocation from the base station. Bandwidth request ranging is performed by only a mobile station which has been already synchronized with the system.
If the base station receives a ranging request message from the mobile station, it generates a connection identifier (CID) of the corresponding mobile station. Afterwards, the base station performs re-negotiation, re-authentication, and re-registration with the base station.
The connection identifier CID means an address for identifying connection mapped into a service flow required between respective peers on a MAC sub-layer. The CID is included in a MAC PDU header. In other words, every connection between the base station and the mobile station is identified by the CID included in the MAC header, and the CID corresponds to a service flow identifier (SFID) that defines quality of service (QoS) parameters of a service flow related to the CID.
A management CID can be allocated from the base station when all mobile stations initially access the base station. In other words, the mobile station can be allocated with the management CID from the base station through a series of ranging and registration processes. The management CID can be divided into basic CID, primary CID and secondary CID.
The basic CID (BCID) is used by the base station to identify the mobile station at an initial access step. In other words, a MAC address of the mobile station is mapped into the basic CID. The primary CID (PCID) can be used for authentication message and dynamic resource allocation messages. Also, the secondary CID (SCID) can be used as option during a capability negotiation procedure of the mobile station.
In addition to the management CID, there are provided a transport CID (TCID) for service of user traffic, a broadcasting CID for broadcasting traffic, and a multicasting CID. In this case, the transport CID (TCID) can be allocated per connection generated between the mobile station and the base station.
Generally, after the mobile station performs handover from a serving base station to a target base station, the mobile station and the target base station can acquire synchronization through handover ranging. However, with the development of the communication technology, a fast handover method for reducing delay is required. In this respect, a method for performing faster handover and reducing a handover ranging process is required.